perf stat: Use affinity for reading

Restructure event reading to use affinity to minimize the number
of IPIs needed.

Before on a large test case with 94 CPUs:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
  3.16    0.106079           4     22082           read

After:

  3.43    0.081295           3     22082           read

Signed-off-by: Andi Kleen <ak@linux.intel.com>

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v2: Use new iterator macros